1. Field of the Invention
The present invention relates to an image signal processor for executing edge enhancement processing and dynamic γ (luminous component) correction processing for an image signal.
2. Related Background Art
For the purpose of enhancing image quality of an image obtained based on an image signal, a television receiver executes various kinds of image signal processing. Edge enhancement processing for enhancing sharpening of an image is frequently used as one of the various kinds of image signal processing.
Conventional edge enhancement processing will hereinafter be described with reference to FIGS. 5, 6A, 6B and 6C.
In edge enhancement processing shown in FIG. 5, the edge enhancement is carried out so as to extract a high frequency band component of an input image signal to be added to the original input image signal. FIGS. 6A, 6B and 6C show waveforms of signals a, b, and c at corresponding points in FIG. 5. For example, when the signal a is inputted to an input terminal 101, an edge detector 102 extracts a high frequency band component like the signal b from the signal a. Then, in an adder 103, the original signal a and the signal b are added to each other to obtain the signal c. By adding the high frequency band component in such a manner, an overshoot and an undershoot are added to an edge portion of an image to enhance an edge of the image.
In addition, in recent years, processing called image signal dynamic γ correction processing has attracted attention. This image signal dynamic γ correction processing is used for detecting characteristics for each scene to carry out gray-level correction corresponding to the scene, thereby realizing gray-level characteristics excellent in contrast. The conventional dynamic γ correction processing will hereinafter be described with reference to FIG. 7.
When an image signal is inputted to an input terminal 301, a scene characteristic detector 302 detects characteristics of the scene. A γ correction curve calculator 303 calculates a γ correction curve based on the detected characteristics to store data on the γ correction curve in a dynamic γ correction RAM table 304. The dynamic γ correction RAM table 304 receives the image signal as an entry and then outputs a signal which is obtained through the dynamic γ correction for the image signal based on the γ correction curve the data on which is stored in the dynamic γ correction RAM table 304. In general, with a technique called histogram flattening processing, the scene characteristic detector 302 detects a histogram of a luminance distribution of a scene as characteristics, and the γ correction curve calculator 303 calculates an accumulated value of the histogram, and carries out its normalization. In addition, an invention is proposed in which not only a histogram, but also a maximum value, a minimum value, an average value, a mode value, a deviation, a black area, and a white area are detected as characteristics of a scene, and a correction amount is controlled, thereby preventing a dynamic range from spreading too much (refer to Japanese Patent Application Laid-open No. H3-126377 for example). In addition, an invention is proposed in which one of previously prepared γ correction curves is selected based on a histogram (refer to Japanese Patent Application Laid-open No. H6-178153 for example) However, mutual effects are not taken into consideration in the edge enhancement processing and the dynamic γ correction processing. Thus, when an edge-enhanced image processing signal is subjected to γ correction-processing, problems as shown in FIG. 8 arise.
1. When the enhanced edge portion is extended through the dynamic γ correction processing, the shoots are also extended, and hence the correction becomes too large correction.
2. When the enhanced edge portion is compressed through the dynamic γ correction processing, the shoots are also compressed, and hence the correction becomes too small correction.
3. When the overshoot and the undershoot are extended and compressed, respectively, the symmetry is lost and hence the image becomes an unnatural image.
In addition, in the case of the dynamic γ correction processing, the preferable gray-level correction characteristics are being currently studied, too. Hence, the γ correction curves having various non-linear characteristics may be used.
The present invention has been made in the light of the above-mentioned circumstances, and it is, therefore, an object of the present invention to obtain excellent edge enhancement effects even when dynamic γ correction processing is executed for an edge-enhanced image signal, and to propose an image signal processor and an image signal processing method in each of which excellent edge enhancement effects can be obtained even for various γ characteristics.